This invention relates to processing of polyester film and more particularly to the improved heat setting of such film.
Polyester film which has been oriented more in the longitudinal or machine direction (MD) than in the transverse direction (sometimes called "tensilized film") has a very high MD tensile strength as well as a high MD tensile strength measured at 5% elongation (usually known as the F-5 value). These properties have made the film very useful as a base for magnetic recording tape. It is highly desirable to produce such a film which is resistant to shrinkage at elevated temperatures, i.e., one which exhibits good thermal dimensional stability. Unfortunately, however, it has been very difficult to produce a film which has high thermal dimensional stability in the MD without some sacrifice of F-5 value since the very processes which tend to increase thermal dimensional stability simultaneously tend to create a film which permits elongation at a lower tensile force. It is also highly desirable to produce a tensilized polyester film having good thermal dimensional stability in the transverse direction (TD).
Campbell, U.S. Pat. No. 3,461,199, discloses a method for improving the MD dimensional stability of polyester film while maintaining the desirably high F-5 value. Briefly stated, the Campbell process involves two-stage heat setting after asymmetrically biaxially stretching polyester film. The instant invention is an improvement over the Campbell process, the teachings of which patent are incorporated herein by reference.
Polyester film produced for magnetic tape base purposes is cast as a wide sheet and processed to achieve the suitable properties. Because the processing includes stretching in both the MD and in the TD, there is a tendency for the polymer molecules to be aligned in a direction somewhat askew from the MD. This phenomenon causes poor tear characteristics in the film resulting in difficulty in slitting very narrow width films. Because this phenomenon can be observed optically by birefringence measurements, it can be referred to as poor birefringence. The term "birefringence bow" is used to describe the situation wherein the birefringence at the edges of the film is different from that in the middle and varies across the width of the film.